Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic devices include organic light emitting devices (OLEDs), organic transistors, organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be an fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
OLED devices are generally (but not always) intended to emit light through at least one of the electrodes, and one or more transparent electrodes may be useful in an organic opto-electronic devices. For example, a transparent electrode material, such as indium tin oxide (ITO), may be used as the bottom electrode. A transparent top electrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, may also be used. For a device intended to emit light only through the bottom electrode, the top electrode does not need to be transparent, and may be comprised of a thick and reflective metal layer having a high electrical conductivity. Similarly, for a device intended to emit light only through the top electrode, the bottom electrode may be opaque and/or reflective. Where an electrode does not need to be transparent, using a thicker layer may provide better conductivity, and using a reflective electrode may increase the amount of light emitted through the other electrode, by reflecting light back towards the transparent electrode. Fully transparent devices may also be fabricated, where both electrodes are transparent. Side emitting OLEDs may also be fabricated, and one or both electrodes may be opaque or reflective in such devices.
Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Photovoltaic (PV) devices or solar cells, which are a type of photosensitive optoelectronic device, are specifically used to generate electrical power. PV devices, which may generate electrical power from light sources other than sunlight, are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term “resistive load” refers to any power consuming or storing device, equipment or system. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no external bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation to generate electric charge carriers.
As used herein, the term “device” is intended to be construed broadly enough to encompass structure such as interconnects that connect other devices to each other.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
Related Art
A number of electronic circuits require low resistance connections between parts that are mechanically separate and/or can move against each other. Examples include: large-area electronics that can be bent or 3-D deformed; printed wire boards with creases along which they can be folded to achieve high density; and integrated circuits that move against their packages under the influence of thermal expansion. Typically, when such movement occurs the electrical contacts between interconnects and circuits are subjected to mechanical stress. If this stress results in mechanical debonding, the circuit also fails electrically.
Photolithographically patterned stretchable interconnects for electrically connecting electronic devices which are supported for movement relative to one another have been described. U.S. patent application No. 2002-0094701 describes a stretchable interconnect formed of a coiled conductor. The coiled conductor is formed by photolithography. In this technique, a negative or positive resist (photoresist) is coated onto an exposed surface of a material. The resist is irradiated in a predetermined pattern, and irradiated (positive resist) or nonirradiated (negative resist) portions of the resist are removed from the surface to produce a predetermined resist pattern on the surface. This can be followed by one or more procedures such as etching, plating, and the like. The coiled conductor is formed of a metal or alloy having a stress gradient extending through the thickness of the conductor. The interconnects become stretchable when a supporting substrate is removed from the interconnect.
Formation of ordered structures in thin films of metals supported on an elastomeric polymer have been described in Bowden, N. et al., Nature, 393, 146 (1998). The ordered structures were spontaneously generated by buckling of thin metal films owing to thermal contraction of an underlying substrate. Films from the vapor phase are deposited on a thermally expanded polymer of polydimethyl siloxane (PDMS). Subsequent cooling of the polymer creates compressive stress in the metal film that is relieved by buckling with a uniform wavelength of 20-50 micrometers. The waves can be controlled and oriented by relief structures in the surface of the polymer to provide intricate ordered patterns. It is described that the patterning process may find applications in optical devices.
Inherent flexibility of thin-film electronics can be used in a variety of applications. One approach to making flexible and deformable structures is to use polymer substrates. The flexibility of the polymer substrate offers application opportunities that utilize curved and/or deformable surfaces. Retina-shaped photosensor arrays described in Hsu, P. et al., Appl. Phys. Lett. 81, 1723-5 (2002), electro-active polymer actuators described in Pelrine, R. et al., Smart Structures and Materials 2001: Electroactive Polymer Actuators and Devices, edited by Y. Bar-Cohen, SPIE Proc. 4329, Bellingham, Wash., (2001) pp. 334-349, or stretchable sensitive skin described in Lumelsky, V. J. et al. IEEE Sensors journal 1, 41 (2001) are electronic systems that combine electronic functions with the flexibility of plastic substrates.
Dielectric elastomer actuators with smart metallic electrodes made of silver were described in Benslimane et al., Smart Structures and Materials 2002, Electroactive Polymer Actuators and Devices, edited by Y. Bar-Cohen, 150 Proceedings of SPIE Vol. 4695 (2002). An elastomer film is spin coat on a mold for forming a corrugated quasisinusoidal profile. Thin metal films are deposited on the corrugated surfaces of the elastomer film. Since the elastomer conserves volume when it is deferred, the electrically-induced stress in the film thickness direction is converted to stress in the direction of actuation. The corrugation depth-to-period ratio is optimized in order to obtain elongation of about 33% before the metal electrode breaks.
An electrode for a bending-electrostrictive polyurethane actuator was described in Watanabe, M. et al., J. Appl. Phys. 92, 4631 (2002). The wrinkled electrode was prepared by in situ deposition of polypyrrole onto a polyurethane elastomer film that was being uniaxially drawn. After the deposition, the film was released from the drawing to make the electrode wrinkle. The bending actuator of the polyurethane film with the wrinkled electrode was improved compared to an unwrinkled one. Polypyrrole is an organic conductor, with an electrical conductivity much lower than that of typical interconnect metals, e.g., gold or aluminum. Accordingly, organic conductors have a greatly restricted applicability compared to metallic conductors.